Mixtures of solid particles can separate or segregate during handling. The non-uniformity of the mixture can result in quality control problems, such as the waste of raw materials, lost production, and increased maintenance and capital costs required to retrofit existing facilities where unwanted segregation of solid particle flows is occurring. Segregation problems can occur with a number of different types of solid particle mixtures, including larger particles, such as coal or rocks, to smaller particles, such as powders, including pharmaceutical powders.
Segregation can occur in a number of different ways, based primarily on various physical properties of the mixture and environmental or handling conditions. Sifting is a prevalent form of segregation. Sifting can be defined as the movement of smaller particles through a mixture of larger particles. This can occur during formation of a pile, as smaller particles percolate into the pile, while coarse particles slide or roll to the perimeter of the pile. In order for sifting segregation to occur, several conditions are required. There must be a difference in particle size, for example, ratios as small as 1.3:1 can induce sifting segregation. Sifting is generally most pronounced when the mean particle diameter is greater than 100 microns. The mixture must be sufficiently free flowing to allow interparticle motion. Finally, there must be movement of the particles relative to one another or portions of the flow within the mixture.
Bulk storage containers, such as hoppers, silos, bunkers and bins, are conventionally used for the storage of quantities of loose particulate solids, including particulate solid mixtures. For the purposes of the present application, the term “hopper” will be used to cover all such differing forms of storage containers for particulate material, where the material fills or partially fills the container and moves during the discharge process to an outlet situated in the lower regions of the container. If all of the material is in motion during discharge, this is referred to as mass flow of the material.
Bulk solids are generally comprised of particles of different sizes. It is commonly desirable to maintain a uniform concentration of each size throughout the mixture during industrial processing, storage, and packaging. However, segregation of the particles by size frequently occurs during processing steps such as the filling or discharge of a hopper. Such actions can lead to segregation by sifting. Accordingly, different regions within a mixture of particulate solids within a hopper can have different proportions of fine and coarse particles. Thus, uniformity of the mixture is lost.
For numerous reasons it can be desirable to be able to handle and move bulk solids with different size particles while maintaining a uniform concentration of each size, including sampling, measurement, and testing processes as well as general handling of the material. Feeder mechanisms can be utilized to supply mixtures of bulk solids to hoppers or other locations.